1. Technical Field
The present invention relates to a method and apparatus for accurately determining in three dimensions information on the location of an object in a passageway and/or the path taken by a passageway, e.g., a borehole. It is more particularly directed to a method and apparatus which uses strain measurements taken from a measurement tool which traverses the passageway to obtain the information.
2. Brief Discussion Of Prior Techniques
The drilling industry has long recognized the desirability of having a position determining system which can be used to guide a drilling head to a predestined target location. There is a continuing need for a position determining system which can provide accurate position information on the path of a borehole and/or the location of a drilling head at any given time as the drill pipe advances. The position information must correspond to a starting location and intended target destination. Ideally, the position determining system should be small enough to fit into a drill pipe in a way which will present minimal restriction to the flow of drilling or returning fluids and accuracy should be as high as possible.
Several prior art systems have been devised to provide such position information. Traditional guidance and hole survey tools such as inclinometers, accelerometers, gyroscopes and magnetometers have been used. One problem facing all of these systems is that they are too large to allow for a "measurement while drilling" of small diameter holes. In a "measurement while drilling" system it is necessary to incorporate a position locator device in the drill pipe, typically near the drilling head, so that measurements may be made without extracting the tool from the hole. The inclusion of such instrumentation within a drill pipe considerably restricts the flow of fluids. With such systems, the drill pipe diameter and the diameter of the hole must often be greater than 4 inches to accommodate the position measuring instrumentation, while still allowing sufficient interior space to provide minimum restriction to fluid flow. Systems based on inclinometers, accelerometers, gyroscopes and magnetometers are also incapable of providing a high degree of accuracy because they are all influenced by signal drift, vibrations, or magnetic or gravitational anomalies. Errors on the order of 1% or greater are often noted.
Some shallow depth position location systems are based on tracking sounds emitted by sonde near the drilling head. In addition to being depth limited, such systems are also deficient in that they require a worker to carry a receiver and walk the surface over the drilling head listening to the sound to track the drilling head location. Such systems cannot be used where there is no worker access to the surface over the drilling head.